FM-CW radar system

ABSTRACT

In an FM-CW radar system, a frequency modulating said modulating wave output from said modulating signal generator has a frequency variation skew with respect to a time axis (modulation skew), and the radar system includes a means for varying the modulation skew by controlling the modulation frequency amplitude or modulation period of the modulating signal. The radar system further includes a means for discriminating a signal component varying in response to the variation of the modulation skew, thereby discriminating a signal related to a target object from other signals. In the case of an FM-CW radar system that performs transmission and/or reception by time division ON-OFF control the radar system includes a means for discriminating a signal which, when the frequency used to perform the ON OFF control is varied, varies in response to the variation of the frequency, thereby discriminating a signal related to a target object from other signals.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No.10/646,905, filed Aug. 22, 2003, which is a divisional of U.S.application Ser. No. 10/148,059, filed May 24, 2002 now U.S. Pat. No.7,002,512, which is the National Stage of International PatentApplication No. PCT/JP01/08397, filed Sep. 26, 2001, which in turnclaims priority of Japanese Patent Application No. 2000-292730, filed onSep. 26, 2000.

TECHNICAL FIELD

The present invention relates to an FM-CW radar system and, moreparticularly, to a system equipped with a means for discriminating asignal related to a target object from noise and a signal from a verydistant target in order not to erroneously detect noise, a very distanttarget, or the like, as a target object.

Background Art

FM-CW radar is used as a radar system for measuring the distance and therelative velocity of a target object. As FM-CW radar can measure thedistance and the relative velocity of a vehicle traveling in front byusing a simple signal processing circuit, and as its transmitter andreceiver can be constructed with simple circuitry, this type of radar isused as an automotive collision avoidance radar.

The principle of FM-CW radar is as follows. An oscillator isfrequency-modulated, for example, by a triangular wave of severalhundred hertz, and the frequency-modulated wave is transmitted; then, areflected signal from a target object is received, and the receivedsignal is frequency-demodulated using the frequency-modulated wave asthe local frequency. Here, the reflected wave from the target is shiftedin frequency from the transmitted signal (i.e., produces a beat)according to the distance between the radar and the target and also tothe Doppler shift due to the relative velocity of the target. Thedistance and the relative velocity of the target object can be measuredfrom this frequency shift.

In an FM-CW radar system, a triangular wave is often used as themodulating signal, and the description given herein deals with the casewhere a triangular wave is used as the modulating signal, but it will beappreciated that a modulating wave of another shape, such as a sawtoothwave or a trapezoidal wave, can be used instead of the triangular wave.

FIG. 1 is a diagram for explaining the principle of FM-CW radar when therelative velocity with respect to the target object is 0. Thetransmitted wave is a triangular wave whose frequency changes as shownby a solid line in part (a) of FIG. 1. In the figure, f₀ is the transmitcenter frequency of the transmitted wave, Δf is the FM modulationamplitude, and Tm is the repetition period. The transmitted wave isreflected from the target object and received by an antenna; thereceived wave is shown by a dashed line in part (a) of FIG. 1. The roundtrip time T of the radio wave to and from the target object is given byT=2r/C, where r is the distance to the target object and C is thevelocity of propagation of the radio wave.

Here, the received wave is shifted in frequency from the transmittedsignal (i.e., produces a beat) according to the distance between theradar and the target object.

The beat frequency component fb can be expressed by the followingequation.fb=fr=(4·Δf/C·Tm)r  (1)

FIG. 2, on the other hand, is a diagram for explaining the principle ofFM-CW radar when the relative velocity with respect to the target objectis v. The frequency of the transmitted wave changes as shown by a solidline in part (a) of FIG. 2. The transmitted wave is reflected from thetarget object and received by an antenna; the received wave is shown bya dashed line in part (a) of FIG. 2. Here, the received wave is shiftedin frequency from the transmitted signal (i.e., produces a beat)according to the distance between the radar and the target object. Inthis case, as the relative velocity with respect to the target object isv, a Doppler shift occurs, and the beat frequency component fb can beexpressed by the following equation.fb=fr±fd=(4·Δf/C·Tm)r±(2·f ₀ /C)v  (2)

In the above equations (1) and (2), the symbols have the followingmeanings.

fb: Transmission/reception beat frequency

fr: Range (distance) frequency

fd: Velocity frequency

f₀: Center frequency of transmitted wave

Δf: Frequency modulation amplitude

Tm: Period of modulated wave

C: Velocity of light (velocity of radio wave)

T: Round trip time of radio wave to and from target object

r: Range (distance) to target object

v: Relative velocity with respect to target object

In an FM-CW radar system, however, there are cases where not only thesignal reflected from the target object but noise and a signal from atarget located at medium or long range are also detected. This can leadto an erroneous detection which indicates that the target object is at adistance different from the actual distance.

An object of the present invention is to provide a radar system which,even in the presence of noise or a signal from a target located atmedium or long range, can identify whether the signal appearing on theradar is the signal from the target object or is noise or a signal fromsome other source, and can thus determine whether the distance to thetarget object has been correctly measured.

DISCLOSURE OF THE INVENTION

In an FM-CW radar system according to the present invention, themodulating wave output from a modulating signal generator has a skewwith respect to the time axis (hereinafter called the “modulation skew”)like a triangular wave, for example, and the radar system includes ameans for varying the modulation skew, wherein the modulation skew isvaried by varying, for example, the amplitude or the period. When themodulation skew is varied, the frequency of the signal related to thetarget object varies in response to the variation of the modulationskew; in view of this, the radar system further includes a means fordiscriminating a signal component varying in response to the variationof the modulation skew, thereby enabling the signal related to thetarget object to be discriminated from other signals.

In the case of an FM-CW radar system that performs transmission and/orreception by time division ON-OFF control, when the frequency used toperform the ON-OFF control is varied, the frequency of the signalrelated to the target object varies in response to the variation of theON-OFF control frequency; in view of this, the time division ON-OFFcontrol type radar system includes a means for discriminating a signalvarying in response to the variation of the ON-OFF control frequency,thereby enabling the signal related to the target object to bediscriminated from other signals.

In a heterodyne FM-CW radar system, there is provided a means fordiscriminating a signal which, when the frequency of an IF signal, i.e.,a downconverted signal, is varied, varies in response to the variationof that frequency, thereby enabling the signal related to the targetobject to be discriminated from other signals.

The modulating signal is a signal in the form of a triangular wave, andthe modulation skew, the transmission/reception switching frequency, orthe IF signal frequency is varied for each pair of the upward anddownward slopes of the triangular wave or every plurality of pairs, orfor each of the upward and downward slopes of the triangular wave.

Further, in an FM-CW radar system that performs transmission and/orreception by time division ON-OFF control, there is provided a means forvarying a pattern, including the duty cycle of the time division ON-OFFcontrol, thereby suppressing signal generation due to targets other thanthe target object.

The frequency modulation is made to vary nonlinearly, for example, inthe form of an arc, with provisions made to discriminate the targetobject based on the frequency distribution of the received signalrelated to the target.

Furthermore, the modulation skew is switched randomly by the modulationskew varying means.

ADVANTAGEOUS EFFECT OF THE INVENTION

According to the present invention, by varying the modulation skew, forexample, in amplitude and in period, by controlling the modulatingsignal and by discriminating the signal component varying in response tothe variation of the modulation skew, it can be easily determinedwhether the received signal is a signal related to the target object.

In the case of an FM-CW radar that performs transmission and/orreception by time division ON-OFF control, the frequency used to performthe ON-OFF control is varied and, by discriminating the signal componentvarying in response to the variation of that frequency, it can be easilydetermined whether the received signal is a signal related to the targetobject.

In the case of a heterodyne FM-CW radar, the frequency of the IF signalis varied and, by discriminating the signal component varying inresponse to the variation of that frequency, it can be easily determinedwhether the received signal is a signal related to the target object.

Further, in the case of an FM-CW radar system that performs transmissionand/or reception by time division ON-OFF control, signal generation dueto targets other than the target object can be suppressed by varying thepattern of the time division ON-OFF control.

The frequency modulation is made to vary nonlinearly and, based on thefrequency distribution of the received signal related to the target, itcan be determined whether the received signal is a signal related to thetarget object.

As described above, according to the present invention, the signal fromthe target object can be discriminated and unwanted signals suppressedwith simple circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining the principle of FM-CW radar when therelative velocity with respect to target object is 0.

FIG. 2 is a diagram for explaining the principle of FM-CW radar when therelative velocity with respect to target object is v.

FIG. 3 is a diagram showing one configuration example of a two-antennaFM-CW radar.

FIG. 4 is a diagram showing one configuration example of asingle-antenna time division ON-OFF control type FM-CW radar.

FIG. 5 is a diagram showing the frequency spectrum of the basebandsignal in the FM-CW radar of FIG. 3.

FIG. 6 is a diagram showing the frequency spectra of the IF signal andbaseband signal, respectively, in the time division ON-OFF control typeFM-CW radar of FIG. 4.

FIG. 7 is a diagram showing the frequency spectra of the IF signal andbaseband signal, respectively, in the time division ON-OFF control typeFM-CW radar of FIG. 4.

FIG. 8 is a diagram showing the frequency spectra of the IF signal andbaseband signal, respectively, in the time division ON-OFF control typeFM-CW radar of FIG. 4.

FIG. 9 is a diagram showing an embodiment of an FM-CW radar according tothe present invention.

FIG. 10 is a diagram showing a triangular wave when the amplitude andperiod of the triangular wave are respectively varied in the FM-CW radaraccording to the present invention.

FIG. 11 is a diagram showing an embodiment of an FM-CW radar accordingto the present invention.

FIG. 12 is a diagram showing how the frequency output from a modulatingsignal generator is varied according to the present invention.

FIG. 13 is a diagram showing an embodiment of a heterodyne FM-CW radaraccording to the present invention.

FIG. 14 is a diagram showing how the frequency output from themodulating signal generator is varied according to the presentinvention.

FIG. 15 is a diagram showing signal processing waveforms in a timedivision ON-OFF control type FM-CW radar.

FIG. 16 is a diagram showing signal processing waveforms in the timedivision ON-OFF control type FM-CW radar according to the presentinvention.

FIG. 17 is a diagram showing transmitted and received waveformsaccording to an embodiment of the present invention.

FIG. 18 is a diagram showing spectral distributions according to theembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in further detail below withreference to drawings. FIG. 3 is a diagram showing one configurationexample of a two-antenna FM-CW radar. As shown, a modulating signalgenerator 1 applies a modulating signal to a voltage-controlledoscillator 2 for frequency modulation, and the frequency-modulated waveis transmitted out via the transmitting antenna AT, while a portion ofthe transmitted signal is separated and fed to a frequency converter 3which functions like a mixer. The signal reflected from a target object,such as a vehicle traveling in front, is received via the receivingantenna AR, and the received signal is mixed in the frequency converter3 with the output signal of the voltage-controlled oscillator 2 toproduce a beat signal. The beat signal is passed through a basebandfilter 4, and is converted by an A/D converter 5 into an digital signal;the digital signal is then supplied to a CPU 6 where signal processing,such as a fast Fourier transform, is applied to the analog signal toobtain the distance and the relative velocity.

FIG. 4 is a diagram showing one configuration example of asingle-antenna time division ON-OFF control type FM-CW radar. As shown,a single antenna ATR is used for both transmission and reception, and atransmit-receive switching device 7 comprising a switching meansswitches between transmission and reception by time division ON-OFFcontrol. At the receiver side are provided a first frequency converter3-1 and a second frequency converter 3-2.

A signal output from the transmit-receive switching device 7 isefficiently radiated into the air from the transmitting/receivingantenna ATR. Reference numeral 8 is a modulating signal generator whichgenerates a modulating signal of frequency fsw for switching thetransmit-receive switching device 7. The signal reflected from thetarget object is received by the transmitting/receiving antenna ATR, andthe received signal is mixed in the first frequency converter 3-1 withthe output of the voltage-controlled oscillator 2 to produce an IFsignal. The signal output from the first frequency converter 3-1 ismixed in the second frequency converter 3-2 with the signal of frequencyfsw generated by the modulating signal generator 8, and downconverted toproduce a signal carrying information on the distance and relativevelocity with respect to the target object.

FIG. 5 is a diagram showing the spectrum of the BB signal passed throughthe baseband filter 4 in the FM-CW radar of FIG. 3.

As shown in FIG. 5, however, noise fn may appear in addition to thesignal fb from the target object, and this noise may be erroneouslydetected as the signal from the target object.

FIG. 6 is a diagram showing the spectrum of the IF signal, i.e., theoutput signal of the first frequency converter 3-1, and the spectrum ofthe BB signal passed through the baseband filter 4 in the time divisionON-OFF control type FM-CW radar of FIG. 4. The output signal of thefirst frequency converter 3-1 in FIG. 4 contains the frequency fsw andits sideband frequencies fsw−fr and fsw+fr, as shown in FIG. 6( a).Here, fsw is the switching frequency of the transmit-receive switchingdevice 7, and fr is the frequency due to the range to the target objectwhen the relative velocity is zero. The greater the distance to thetarget object, the farther the sideband frequencies are spaced away fromfsw. This output signal is mixed in the second frequency converter 3-2with the signal of frequency fsw and downconverted to a frequency equalto the difference between the frequencies fsw and fsw±fr to extract thesignal fb, which is passed through the BB filter and fed as the BBsignal to the A/D converter 5. At this time, however, a noise signal fnmay appear in the vicinity of the switching frequency fsw in the IFsignal frequency band, as shown in FIG. 6( a). In that case, the noisesignal directly enters the BB band and appears as fn1, or isdownconverted and appears as fn2 in the BB band.

FIG. 7 is a diagram showing the spectrum of the IF signal, i.e., theoutput signal of the first frequency converter 3-1, and the spectrum ofthe BB signal passed through the baseband filter in the time divisionON-OFF control type FM-CW radar of FIG. 4. As shown in FIG. 7( a), ahomodyne component of a signal from a medium-range target, which is notthe target object, enters the IF signal frequency band and appears assignal fh which, in the beat signal band, appears as signals fh1 andfh2. In this case, these signals are eliminated by the BB filter sincetheir frequencies are higher than the BB band.

FIG. 8 is a diagram showing the spectrum of the IF signal, i.e., theoutput signal of the first frequency converter 3-1, and the spectrum ofthe BB signal passed through the baseband filter in the time divisionON-OFF control type FM-CW radar of FIG. 4. As shown in FIG. 8( a), whenthere is a long-range target, its homodyne component enters the IFfrequency band and appears as signal fh. This signal appears as signalfh1 in the beat signal band and as signal fh2 in the BB band, as shownin FIG. 8( a). In this case, the signal fh1 is eliminated by the BBfilter as the frequency is higher than the BB band. However, the signalfh2 is not eliminated by the BB filter, and this signal, though it is anoise component, may be erroneously detected by determining that thereis a target object at a distance nearer than it actually is.

FIG. 9 is a diagram showing an embodiment of an FM-CW radar systemaccording to the present invention. The configuration is the same asthat of FIG. 3, except for the inclusion of a modulating signalgenerator control unit 10. In this figure, the control unit 10, underthe control of the CPU 6, variably controls the skew in, for example,amplitude or period, of the modulating signal to be output from themodulating signal generator 1.

First, the present invention will be described by dealing with the caseof variably controlling the amplitude of the modulating signal. Aspreviously described with reference to FIG. 1, when the relativevelocity with respect to the target object is 0, the frequency of thetransmitted wave changes as shown by the solid line in part (a) ofFIG. 1. The transmitted wave is reflected from the target object andreceived by the antenna, the received wave being shown by the dashedline in part (a) of FIG. 1. Here, the received wave is shifted infrequency from the transmitted signal (i.e., produces a beat) accordingto the distance between the radar and the target object. The beatfrequency component fb can be expressed by equation (1) as previouslydescribed.fb=fr=(4·Δf/C·Tm)r  (1)

From equation (1), it will be noted that Δf represents the frequencymodulation amplitude, and that Δf can be varied by varying the amplitudeof the modulating signal. For example, when the amplitude of themodulating signal is doubled, Δf is doubled and, from equation (1), fbis also doubled. FIG. 10 is a diagram showing a triangular wave used asthe modulating signal when its amplitude is varied. Part (a) shows thetriangular wave with the normal amplitude (equivalent to Δf), and part(b) shows the triangular wave with its amplitude doubled (equivalent to2Δf).

In the FM-CW radar system of FIG. 9, when the amplitude of themodulating signal is varied by n times by controlling the modulatingsignal generator 1 from the control unit 10, the value of the beatfrequency component fb varies by n times, as described above. As shownin FIG. 5, the received signal contains the noise signal fn as well asthe signal fb from the target object. Here, by controlling themodulating signal generator 1 from the control unit 10, the amplitude ofthe triangular wave frequency is varied to vary Δf by n times. As aresult, the frequency fb of the signal from the target object varies byn times in response to the variation of Δf. However, as the frequency fnof the noise signal remains unchanged, it becomes possible todiscriminate which signal is the signal from the target object. Thisdiscrimination is done by the CPU 6 in the FM-CW radar. Thediscrimination described below is also done by the CPU 6.

Next, a description will be given of the case of variably controllingthe period of the modulating signal.

From equation (1), it will be noted that Tm represents the period of themodulating signal. Accordingly, when the period Tm of the modulatingsignal is varied, for example, by n times, the beat frequency componentfb varies by 1/n times. FIG. 10( c) is a diagram showing a triangularwave used as the modulating signal when its period is varied. Part (a)shows the triangular wave with the normal period Tm, and part (c) showsthe triangular wave with a period nTm which is n times the normal periodTm.

In the FM-CW radar system of FIG. 9, when the period of the modulatingsignal is varied to the period nTm, n times the normal period Tm, bycontrolling the modulating signal generator 1 from the control unit 10,the value of the beat frequency component fb varies by 1/n times. Here,by controlling the modulating signal generator 1 from the control unit10, the period of the triangular wave frequency is varied to vary Tm byn times. As a result, the frequency fb of the signal from the targetobject varies by 1/n times in response to the variation of Tm. However,since the frequency fn of the noise signal remains unchanged, it becomespossible to discriminate which signal is the signal from the targetobject.

As shown in FIG. 2, when the relative velocity with respect to thetarget is v, the frequency of the transmitted wave changes as shown bythe solid line in part (a) of FIG. 2. The transmitted wave is reflectedfrom the target object and received by the antenna, the received wavebeing shown by the dashed line in part (a) of FIG. 2. Here, the receivedwave is shifted in frequency from the transmitted signal (i.e., producesa beat) according to the distance between the radar and the targetobject. The beat frequency component fb can be expressed by equation (2)as previously described.fb=fr±fd=(4·Δf/C·Tm)r±(2·f ₀ /C)v  (2)

In this case also, by noting Δf or Tm, the amplitude or period Tm of themodulating signal is varied using the control unit 10; then, as the beatfrequency component fb varies correspondingly, it becomes possible todiscriminate which signal is the signal reflected from the targetobject. The frequency component fb consists of the range frequencycomponent fr and velocity frequency component fd, of which only therange frequency component fr varies in the above case. However, sincethe frequency component fb varies as a whole, the signal from the targetobject can be discriminated.

The above embodiment has been described by dealing with the case wherethe present invention is applied to a two-antenna type FM-CW radar, butthe invention is also applicable to a single-antenna type FM-CW radar.

FIG. 11 is a diagram showing an embodiment of an FM-CW radar systemaccording to the present invention. This embodiment concerns asingle-antenna time division ON-OFF control type FM-CW radar to whichthe present invention is applied. The configuration of FIG. 11 is thesame as that of FIG. 4, except for the inclusion of a modulating signalgenerator control unit 11 for the modulating signal generator 8. In thisfigure, the control unit 11, under the control of the CPU 6, variablycontrols the frequency (period) of the modulating signal to be outputfrom the modulating signal generator 8. As a result, the ON-OFFfrequency (period) of the transmit-receive switching device 7 changes,and the frequency applied to the second frequency converter 3-2 alsochanges. Since the switching frequency fsw changes, the sideband signalfrequencies fsw−fr and fsw+fr shown in FIGS. 6 to 8 also change, so thatthe signal from the target object can be discriminated.

The frequency (period) of the modulating signal output from themodulating signal generator 8 is varied, for example, as shown in FIG.12. In this case, the frequency is varied in synchronism with thetriangular wave being output from the other modulating signal generator1. In example 1 of FIG. 12, the frequency is varied as fsw1, fsw2, andfsw3 in sequence for each up/down cycle of the triangular wave. As aresult, the ON-OFF switching frequency fsw changes, and fsw−fr andfsw+fr also change accordingly. On the other hand, other frequencycomponents such as noise remain unchanged, so that the signal from thetarget object can be discriminated from other signals. In the aboveembodiment, the frequency is varied in sequence for each up/down cycle,but the frequency may be varied every plurality of up/down cycles. Inthe latter case, the frequency may be varied randomly.

In example 2 of FIG. 12, the frequency of the modulating signal outputfrom the modulating signal generator 8 is varied for each half cycle (upor down) of the triangular wave. In this case, the frequency of thesignal from the target object varies for each half cycle (up or down) ofthe triangular wave.

FIG. 13 is a diagram showing a two-antenna heterodyne type FM-CW radarsystem. Though the radar system will be described here as being thetwo-antenna type, the basic principle is the same for the single-antennatype. The system shown here differs from the configuration of FIG. 11 inthat two antennas, the transmitting antenna AT and the receiving antennaAR, are provided and the transmit-receive switching device is omittedbecause of the two-antenna system. In addition to that, an up converter9 is provided between the voltage-controlled oscillator 2 and the firstfrequency converter 3-1 so that the frequency of the signal to be inputto it from the modulating signal generator 8 can be controlled by themodulating signal generator control unit 11. The up converter 9 takes asinputs the signal of frequency f₀ from the voltage-controlled oscillator2 and the modulating signal of frequency If1 from the modulating signalgenerator 8, and outputs a signal of frequency f₀+If1 as the localsignal to the first frequency converter 3-1. In this case also, when thefrequency If1 of the signal output from the modulating signal generator8 is varied, the signal frequencies fsw(If1)−fr and fsw(If2)+fr shown inFIGS. 6 to 8 also vary, causing the beat frequency component fb to varyaccordingly, so that the signal from the target object can bediscriminated.

The frequency (period) of the modulating signal output from themodulating signal generator 8 is varied, for example, as shown in FIG.14. In this case, the frequency is varied in synchronism with thetriangular wave being output from the other modulating signal generator1. In example 1 of FIG. 14, the frequency is varied as If1, If2, and If3in sequence for each up/down cycle of the triangular wave. As a result,the signals Ifn+fr and Ifn−fr change at the output end of the firstfrequency converter 3-1, but since the noise frequency remainsunchanged, the signal from the target object can be discriminated fromother signals. In the above embodiment, the frequency is varied insequence for each up/down cycle, but the frequency may be varied everyplurality of up/down cycles. In the latter case, the frequency may bevaried randomly.

In example 2 of FIG. 14, the frequency of the modulating signal outputfrom the modulating signal generator 8 is varied for each half cycle (upor down) of the triangular wave. In this case, the frequency of thesignal from the target object varies for each half cycle (up or down) ofthe triangular wave.

FIGS. 15 to 16 are diagrams for explaining an embodiment in which thepresent invention is applied to the time division ON-OFF control typeFM-CW radar. This embodiment will be described by referring to the timedivision ON-OFF control type FM-CW radar system shown in FIG. 11.

FIG. 15 is a diagram showing signal processing waveforms in aconventional art time division ON-OFF control type FM-CW radar. In thefigure, part (a) shows a waveform defining the switching timing of thetransmit-receive switching device 7; the signal Ssw shown here is outputfrom the modulating signal generator 8. Part (b) shows a waveform Tondefining the transmission ON timing based on Ssw, and (c) a waveform Rondefining the reception ON timing based on Ssw. On the other hand, part(d) shows a waveform SA illustrating the return timing of thetransmitted signal upon reflection, and (e) a waveform SB illustratingthe timing for the reflected signal to be received by the radar when thereception is ON.

As can be seen, the waveform SA is delayed in timing with respect to thewaveform Ton by an amount equal to the round trip time from the radar tothe target object and back to the radar. For example, the time intervalT between the pulse shown by oblique hatching in the waveform Ton andthe pulse shown by oblique hatching in the waveform SA is 2r/C, where ris the distance between the radar and the target object and C is thevelocity of light. When the target is at a far distance, the pulse shownby horizontal hatching in the waveform SA, for example, is returned forthe oblique hatched pulse in the waveform Ton. The pulse time intervalT′ in this case is 2r′/C.

FIG. 16 is a diagram for explaining the embodiment of the presentinvention. In the present invention, provisions are made not to receivea reflected wave from any target other than the target object, byturning off the receiving gate when a reflected wave from a medium-rangeor long-range target, which is not the target object, is returned. Toachieve this, in the present invention, a transmission/reception OFFperiod Toff is provided in the signal Ssw as shown in FIG. 16( a). Thisresults in the formation of a transmission OFF period Ton-off and areception OFF period Ron-off in Ton and Ron, respectively. As a result,when a signal transmitted, for example, with the timing of the obliquehatched pulse in Ton is returned by being reflected on a very distanttarget, a pulse shown by dashed lines appears in the waveform SA show inpart (d), but at this time, as the gate of Ron is closed, the returnsignal is not received; in this way, the unwanted signal from the verydistant target can be eliminated. By varying the transmission/receptionpattern in this way, it becomes possible to suppress signal generationdue to a medium-range or long-range target which is not the targetobject.

FIG. 17 is a diagram for explaining an embodiment of the presentinvention. FIG. 17( a) shows the transmitted waveform of the FM-CWradar. In the conventional art, the transmitted waveform is triangularas shown in FIG. 1( a). On the other hand, in the present invention, thetransmitted wave has a nonlinear shape with the linearity of theconventional art waveform degraded as illustrated here; in the exampleshown, the waveform is shaped in the form of an arc to make thefrequency deviation of the rectangular wave nonlinear.

FIG. 18( b) shows the transmitted and received waveforms according tothe conventional art. These waveforms are the same as those shown inFIG. 1( a). In this case, the frequency difference fr between thetransmitted and received waves is the same at any point in time.

In contrast, in the present invention, as the linearity of thetransmitted wave is degraded, the linearity of the received wave is alsodegraded as shown in FIG. 17( c). As a result, the frequency differencefr between the transmitted and received waves varies with time. Forexample, the frequency difference fr1 in the first half of the risingportion of the wave differs from the frequency difference fr2 in thesecond half of the rising portion, as shown, that is, fr1>fr2. Thedifference between fr1 and fr2 increases with increasing distance to thetarget. Using this characteristic, a signal from a very distant targetcan be distinguished and eliminated from the target objects.

FIG. 18 shows frequency spectra detected. As shown, when the target isnear, the spectrum exhibits a distribution such as shown by a, whilewhen the target is distant, the spectrum exhibits a distribution such asshown by b.

Accordingly, when the detected spectrum has a distribution such as shownby b, the detected target can be determined as being a very distanttarget and be eliminated.

In the above embodiment, the configuration shown in FIG. 9, for example,can be used for the FM-CW radar. Further, the triangular wave need notnecessarily be shaped in the form of an arc as shown above, but may beshaped in any suitable form as long as it causes a difference betweenfr1 and fr2.

1. A heterodyne FM-CW radar system which frequency-modulates avoltage-controlled oscillator by applying thereto a modulating signalfrom a first modulating signal generator, and which transmits afrequency-modulated wave and receives a reflected wave, wherein saidsystem includes a second modulating signal generator for generating adownconverting signal (IF signal), a modulating signal generator controlunit for varying the frequency of the second modulating signal generatorand a signal processing unit for discriminating a signal that variesfrom a signal that does not vary in response to the variation of thefrequency of said IF signal when said frequency is varied, wherein saidsignal processing unit discriminates a signal component varying inresponse to the variation of frequency as a signal component related toa target object, and discriminates a signal component not varying inresponse to the variation of frequency as a signal component not relatedto a target object.
 2. A heterodyne FM-CW radar system as claimed inclaim 1, wherein said modulating signal is a signal in the form of atriangular wave, and said means for varying the frequency of said IFsignal varies said frequency for each pair of upward and downward slopesof said triangular wave or every plurality of said pairs.
 3. Aheterodyne FM-CW radar system as claimed in claim 1, wherein saidmodulating signal is a signal in the form of a triangular wave, and saidmeans for varying the frequency of said IF signal varies said frequencyfor each of upward and downward slopes of said triangular wave.